You can thin the crown of a mature rowan, but I'd think very carefully before doing it - the shape of the rowan is one of it's chief glories and it's very easy to get these things wrong and end up with a stubby horror which will never improve.

As Welshonion says, why do you feel it needs reducing? Is there another way around the problem?

How big is it? Now would be the time if you are going to do it. I have one that I keep to about 12 - 15 ft. I usually take out any cross pieces, damaged branches and then keep the leaders cut back. About every 3 to 4 yrs. Let the birds have the berries first though.

I'd be very surprised if a healthy Mountain Ash were 'unsteady' even in a strong wind. They're a tree which is very much at home on windy hillsides, and as such are flexible and bendy in the wind - it is this flexibility which gives them their strength - it is more rigid types of trunk that snap in the wind.

If it is moving at the base, then it may well be rotten at the roots and needs removing.